The u. s. has advocated strengthening oil embargoes against iran because

The U.S. has advocated strengthening oil embargoes against Iran because of Iran's ambition to produce a nuclear weapon.

Other sanctions on Iran were initiated by the United States for the purpose of:
1) blocking the transfer of weapons, components, technology, and dual-use items to Iran's prohibited nuclear and missile programs.
2) targeting select sectors of the Iranian economy relevant to its proliferation activities
3) inducing Iran to engage constructively through discussions with USA, China, France, Germany, the UK, and Russia in the to fulfill its nonproliferation obligations.

Iran's full compliance with its international nuclear obligations would open the door to its receiving treatment as a normal non-nuclear-weapon state under the Nonproliferation treaty and the sanctions imposed will be lifted